Electrochemical elements such as lithium ion cells generally have composite electrodes containing particles of electrochemically active constituents, a binder and also current collectors. The binder ensures mechanical stability of the electrode; in particular, it should ensure contact of the particles between one another and with the current collector. Decontacting both between individual electrochemically active particles and between the particles and the current collector can occur, for example, due to gas formation in the electrode as a result of decomposition of the electrolyte or due to the electrochemically induced volumetric dynamics of an electrode. Decontacting is frequently associated with gradually increasing decreases in capacity, which can ultimately lead to the electrode concerned becoming unusable.
Binders based on fluorinated polymers and copolymers, in particular binders based on polyvinylidene fluorides (PVdF) and polyvinylidene fluoride-hexafluoropropylene (PVdF-HFP) are known. Electrodes having such binders are described, for example, in EP 1 261 048 and U.S. Pat. No. 5,296,318.
EP 14 89 673 discloses electrodes having a binder based on styrene-butadiene rubber. In the production of the electrodes, sodium carboxymethylcellulose is used to adjust the viscosity.
In an electrode, the polymers or copolymers used as binder usually form a matrix in which the electrochemically active materials are present in finely dispersed form. Commercial electrochemically active materials such as graphite are generally completely insoluble in the polymers and copolymers mentioned. They do not form a firm bond with the binder matrix. Rather, physical attachment, e.g., via adhesion forces, or mechanical attachment occurs. Decreases in capacity as a result of decontacting can in the case of such electrodes frequently be measured after only a few charging and discharging cycles.
From an ecological and economic point of view, the use of the above-mentioned partially fluorinated polymer binders, in particular, is problematic since processing these binders requires the use of organic solvents such as N-methylpyrrolidin-2-one or acetone. Corresponding safety measures and occupational hygiene measures are necessary. The polymer binders dissolve at least partially in these solvents and can then readily be processed further. However, N-methylpyrrolidin-2-one in particular is suspected of being harmful to health. The solvents generally have to be incinerated after use, which results in an increase in carbon dioxide emissions.
It could therefore be helpful to provide batteries which are superior to known batteries in terms of their capacitive performance and their life. In particular, the capacitive performance should decrease less sharply with increasing age of the battery than is the case for known batteries.